I think Ruby as a given name for a boy, no, but as a nickname for Reuben/Rubin it's gorgeous and works perfectly. Yes most Rubys he would come across would be girls, but he would have Rubin on the register which is quite obviously a boy's name he'd be fine in class and that. Plus Ruby on a male is not unheard of - I don't know where you're from, but there's an Irish jockey called RubyWalsh who's pretty well known in the UK and Ireland at least. His full name's Rupert but he goes by Ruby professionally. So if anyone questions him, he's got a male Ruby to cite.